For many years, plethysmography has been used as a laboratory method for the assessment of the integrity of the vascular bed of the limbs by observing changes in peripheral blood volume. Within the last few years, the technique of using photoplethysmography has developed considerably. The technique involves the use of an optical source (usually an incandescent lamp) and an appropriate detector, and is theoretically based on the large difference between the extinction coefficients of blood and the body tissues. (See "Photoplethysmography" by J. Weiman, Manual of Psychophysiological Methods, North Holland Publ. Co., Amsterdam, Holland, 1967). The region of 7000-9000 A appears to be optimum for optical transmission through tissue and blood with large differences in optical transparency of tissue and blood. At 8050 A for example, Kramer ("Influence of Oxygen Saturation, Erythrocite Concentration and Optical Depth, Upon the Red and Near Infrared Regions", Kramer, K., Elam, J. O., Saxton, G. A., and Elam, W. N., Amer. J. Physiol. 165, pp. 229-246, 1951) has stated that normal whole blood layer of 1.3 mm thickness would transmit 0.7% of incident light; a similar tissue layer would yield a value of 62%.
In attempting to assess Raynoud's Phenomenon of occupational origin (also known as vibration white fingers syndrome (VWF), Taylor and others ("Proceedings of the International Occupational Hand-Arm Vibration Conference", edited by Wasserman, D., Taylor, W., and Curry, M., DHEW (NIOSH) Publication No. 77-170, April, 1977) have stated that there presently does not exist a single comprehensive and reliable test for assessing VWF in either the clinical or field situation. In a recently conducted laboratory study, these authors have successfully used single channel photoplethysmography as a tentative means of distinguishing between the peripheral circulation of normal human subjects and diagnosed Raynaud's disease patients.
A preliminary search has revealed the following prior patents which appear to show the state of the art:
Smith, U.S. Pat. No. 3,103,214
Planyi et al, U.S. Pat. No. 3,628,525
Page, U.S. Pat. No. 3,841,314
Heule, U.S. Pat. No. 3,980,075
Kenoshi et al, U.S. Pat. No. 3,998,550
Benjamin, U.S. Pat. No. 4,015,595